This invention relates to serial digital data streams consisting of so-called ones and zeroes. In the past, many methods have been used to extract clock signals from a serial data stream. Most of these methods require a given number of ones in a string of data, and a guaranteed ones density. In a tuned-tank system, the tank is excited by the ones in the data signal and allowed to free-run during the zeros. The output of this is usually converted to a digital clock signal using a comparator or some such device. The amplitude of the tank signal begins to decay exponentially during a long string of zeros. Eventually, the amplitude becomes so small that a coherent clock cannot be extracted.
The Q (Quality) of the tank determines how long the clock can be maintained during a string of zeros. The Q of the tank also determines how sharply tuned the tank is. For an incoming data stream, the clock rate must be limited to a range of frequencies in order for the tank to be excited by the data stream. So there is a trade-off between making the Q large enough to "bridge" over a predetermined string of zeros, and small enough to accommodate a given range of data clock rates. Also, as the amplitude of the tank circuit decays during a string of data zeros, the frequency of the tank signal wanders back to its resonant frequency from the input data clock rate. Since the recovered clock is usually used to clock in the data into a serial in, parallel out shift register, the phase of the clock becomes somewhat critical. As the frequency of the recovered clock wanders back to its resonant frequency, the phase of the clock with respect to the data changes.
Thus, a principal problem with tuned-tank clock recovery circuits is loss of clock recovery when a high number of zeros is received. The consequences of this occurrence are that the framing pattern embedded in the data may be lost, resulting in a large number of bit errors in the received data while the framer IC searches for the framing pattern. As a result, there is a need for a clock recovery circuit that is self-regenerative to avoid loss of clock yet is flexible enough to track an incoming data stream clock rate.